Observation of microtremors and earthquake ground motion of aftershocks of 2014 Northern Nagano earthquake for estimation of site amplification

Heavy damage was experienced in Hakuba village, Nagano prefecture during the 2014 Nagano-ken Hokubu earthquake, Japan. In particular many wooden houses were collapsed in Horinouchi and Mikkaiichiba district in the village. Characteristics of ground shaking during the main shock are not known in the damaged area due to no availability of strong motion records. It has been known that this area was suffered in past several earthquakes. This suggests that site amplification may be one of the reasons for the heavy damage during the earthquake. We conducted aftershock observations and microtremor explorations at 13 stations in and around the damaged area to estimate site effects. Ground motions from aftershocks with magnitudes of less than 4 are dominant at periods of 0.5 to 1 second in the damaged area, while those in non-damaged areas are rich in much high frequency components. The microtremor array explorations at the stations revealed shallow S-wave velocity profiles donw to 100 meters. Shallow soft soil layers over stiff soil having an S-wave velocity of 500m/s are responsible for the amplification of earthquake ground motions. The comparison of seismic intensity estimated from the aftershock records with average velocity in the top 30 meters indicated that the seismic intensity at the damaged area can be about 6.4.